This invention relates to release coatings for substrates. In one of its aspects this invention relates to improved low release value coatings comprising a mixture of (a) polyorganosiloxanes having silicon-bonded hydroxyl groups on the ends of the molecule, and (b) polymethylhydrogensiloxanes having an average of at least 3 SiH groups per molecule. In another aspect of the invention there is provided a high release value coating composition comprising a mixture of (a) and (b) plus a polyorganosiloxane containing triorganosiloxy end groups. In another of its aspects this invention relates to differential release papers or laminates coated on one side with a high release value polyorganosiloxane coating and on the opposite side with a lower release value of polyorganosiloxane coating. In still another of its aspects this invention relates to improving the shelf life of differential release papers or laminates by coating one side with a high release value of polyorganosiloxane coating and the opposite side with a lower release value of polyorganosiloxane coating.
In one of its concepts this invention relates to the prevention of migration of high release silicones from one surface of a differential release paper to a low release value silicone coated surface brouoght into contact with the high release value silicone coating. The migration is reduced by the method of curing the high release value silicones which provides a relatively more stable coating composition.
The invention concerns a differential release paper or laminate consisting of a substrate such as paper which can have been coated on one or both sides with a polymer such as polyethylene. The resulting laminated structure is then coated on one side with a silicone release agent after which the second side is coated with a silicone release agent designed to give a dissimilar release value from that of the first side.
Problems usually arise, however, in adequately curing each silicone overcoate and in the tendency of adjacent surfaces of stacks or rolls to transfer coating components between each other. When transfer occurs the release value of the higher value release agent is lessened so that the difference between the release value of the two sides of the differential release paper is lessened or, at worst, the values become about the same, so that the advantages of a differential release paper are nullified.
The laminates referred to in the invention are normally made at the rate of several hundred feet per minute and after coating with the silicone solutions are passed through ovens to remove solvent and at least initiate curing of the applied resins. The laminate is then usually wound into a roll and stored until ready for use. It is during this storage time that transfer between the adjacent sides can occur. Of course, sheets of an uncoated dissimilar material can be employed as spacer sheets to prevent contact between the rolled or stacked sheets to remedy the transfer, but this practice is time consuming and expensive.
I have found that by the use of a dual catalyst system in preparing the silicone release coating compositions of this invention for coating a substrate a release coating of improved stability is obtained.
It is therefore an object of this invention to provide a coating composition that can be used to produce a high release value coating on a substrate. It is another object of this invention to provide a method for producing a high release value coating on a substrate and the product of improved stability produced thereby. It is another object of this invention to provide a differential release paper or laminate of improved shelf life.